Generally, as technique for forming a thin film on a hard material to be printed by printing, lithography offset printing or the like is known. But in the lithography offset printing, usable printing ink needs to be faithfully transferred to a surface plate without being emulsified with dampening water in a great amount. Therefore, usable ink is limited to printing ink having a high viscosity of 80,000-100,000 c.p.s. and a film formed by ink transfer lacks smoothness.
Therefore, a thin film cannot be formed by printing if it is necessary to form a film superior in smoothness by printing ink to be used to form a liquid crystal orientation film or a liquid crystal enclosing seal and having a low viscosity of several tens -30,000 c.p.s.
In order to overcome this issue, the applicant proposed a thin film forming apparatus (Japanese Laid-Open Utility Model Publication No. 58-170864) as shown in FIG. 14. In this apparatus, a flat intaglio 42 having recesses consisting of a great number of small openings and grooves and a material to be printed 43 on which a thin film is to be formed are arranged on a base 41, and a printing roll supporting frame 45 having an ink supply device (not shown) for charging ink having a viscosity of several tens to 30,000 c.p.s. into the recesses of the intaglio 42 and a printing roll 44 having an elastic letterpress 44a consisting of rubber or resin is movable between the intaglio 42 and the material 43 with respect to the base 41. A doctor, for scraping off an excessive amount of ink from the recesses of the intaglio 42 is provided on the printing roll supporting frame 45.
In the above-described apparatus, ink is supplied from the ink supply device to the recesses of the intaglio 42; an excessive amount of ink is scraped off with the doctor 46; then, the printing roll 44 is rotated by pressing the printing roll 44 against the intaglio 42; ink in the recesses of the intaglio 42 is applied to the surface of the elastic letterpress 44a mounted on the printing roll 44; the printing roll 44 having the ink applied thereto is fed over the material 43; and the ink on the elastic letterpress 44a is transferred to the surface of the material 43 while the printing roll 44 is rotating. Thus, a predetermined pattern is formed thereon.
The applicant also proposed a thin film forming apparatus (Japanese Laid-Open Patent Publication No. 62-202736) in which the intaglio 42 for supplying a certain amount of ink to the printing roll 44 is not constituted as a flat plate but as a cylindrical member.
According to these thin film forming apparatuses, ink is transferred to the surface of the material according to the arrangement of the recesses of the intaglio, and the surface of the thin film thus formed has regular projections and recesses called "gravure trace" having a configuration similar to those of the intaglio. That is, when ink charged in the recesses of the intaglio is transferred to the surface of the printing roll, the ink is transferred to the printing roll in conformity with the arrangement of the projections and recesses of the intaglio, and when the ink is transferred to the material, an ink film having projections and recesses in conformity with the arrangement of those of the intaglio is also formed on the material. Ink flattens to some extent on the printing roll and the material but the smoothness of the thin film of ink formed thus has gravure trace and is not preferable.
The pattern of the ink film is formed in correspondence to the pattern of the projection of the printing roll. The gravure trace of the ink film is formed in the pattern edge of the projection and the pattern edge of the ink film transferred to the material is indented due to the gravure trace and lacks linearity.
Such a phenomenon occurs outstandingly when the edge of the pattern is intended to be clear by reducing the contact pressure between the printing roll and the intaglio as well as the material, when the viscosity of ink is a little high, when a gravure screen used to manufacture the intaglio is rough in order to increase a film thickness, or when a fine pattern such as a line or a dot is printed.
It is, therefore, an object of the present invention to resolve the above-described issue and provide a thin film forming apparatus capable of forming, on a material to be printed, a highly accurate thin film smooth and having an edge preferable in linearity by transferring a smooth and uniform ink film to the material.